The present invention relates to a method for treating unfired (green) ceramic cores for use in casting molten metallic materials.
Most manufacturers of gas turbine engines are evaluating advanced investment cast turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. Internal cooling passages are formed in the cast airfoils using one or more thin airfoil shaped ceramic cores positioned in a ceramic shell mold where the molten metal is cast in the mold about the core. After the molten metal solidifies, the mold and core are removed to leave a cast airfoil with one or more internal passages where the cores formerly resided.
The ceramic core is typically made using a plasticized ceramic compound comprising ceramic flour, organic thermosetting and/or thermoplastic binder and various additives. The ceramic compound is injection molded or transfer molded at elevated temperature in a core die or mold. When the green (unfired) core is removed from the die or mold, it typically is placed between top and bottom setters to cool to ambient temperature before core finishing and gauging operations and firing at an elevated sintering temperature.
The green core can exhibit distortion from stresses induced in the core from the molding and/or ambient cooling operations. Distortion can be a particular problem with respect to the airfoil region of the core have a trailing edge with a relatively thin cross-section that is prone to distortion. As a result, the green ceramic cores can exhibit dimensional variations from one core to the next in a production run of cores. Moreover, the green core may be improperly contacted by the top or bottom setter such that dimensional variations from one core to the next occur in a production run.
An object of the present invention is to provide a method of treating an unfired ceramic core in a manner to reduce distortion of the core and improve yield of cores that meet dimensional tolerances.
In one embodiment of the invention, a method for treating an unfired ceramic core comprises placing an unfired (green) ceramic core having a molded core shape and a binder on at least one setter, placing the setter and the green ceramic core thereon on a conveyor, conveying the setter and the green core through the heating oven to heat the setter and the green ceramic core to an elevated superambient temperature. Heating of the green ceramic core in this manner conforms the core to a surface of the setter to reduce distortion of the core and improve yields of cores within preselected dimensional tolerances. To this end, the setter and the green ceramic core preferably are heated to a superambient temperature at or above a softening temperature of the binder present in the molded green core. Each of a plurality of green ceramic cores can be treated by placing the core on respective setter and placing each core/setter on the conveyor for transport through the heating oven one after another or side-by-side on the conveyor.
In a particular embodiment of the invention, the rate of travel of the conveyor through the heating oven is controlled such that the setter and the green ceramic core are heated to the desired superambient temperature proximate an exit opening of the heating oven. The setter and the green ceramic core are removed from the conveyor after exiting the heating oven so that the setter and the green ceramic core can cool to ambient temperature. The setter supplies heat to the green ceramic core after exiting the heating oven and during cooling to ambient temperature is advantageous to reduce the time needed to heat the green ceramic core in the heating oven.
In a preferred embodiment of the invention, the green ceramic core is placed between a top setter and a bottom setter and is conveyed through the heating oven between the top setter and bottom setter.
The invention is beneficial for, although not limited to, treating a green ceramic core that includes an airfoil region having a trailing edge with a relatively thin cross-section that is prone to distortion after removal from a core molding die.